Circuitry for processing signals occurring in a heterodyne interferometer

ABSTRACT

A signal processing circuit for a reference signal occurring in a heterodyne interferometer and a measured signal. The underlying frequency modulation of the radiation source of the heterodyne interferometer results in phase jumps in both signals. Signal filtering of both the reference signal and the measured signal with a gate signal removes from both signals those signal components that have the same phase sign. Further simplification of the signal processing results from signal interpolation by band pass filters and signal down-mixing into a lower frequency range below the heterodyne frequency. The input signals processed by the circuit can be further processed by a conventional phase comparator.

FIELD OF THE INVENTION

The present invention relates to a signal processing circuit forreference and measured signals occurring in a heterodyne interferometeraccording to the definition of the species of the independent claim.Heterodyne interferometry is used, for example, for contactlessdistance, angle, or velocity measurements. An optical measuring beam isdirected onto grid structures or mirrors placed on the measured object.The surface of the measured object may occasionally even serve as adiffraction grid or a reflector.

BACKGROUND INFORMATION

European Patent Application No. 0 461 119 (corresponding toInternational Patent Publication No. WO 09/10195) describes a heterodyneinterferometer in which a laser diode is used as the radiation source.The injection current of the laser diode is periodically modulated usinga saw tooth or triangular modulation signal, for example. Injectioncurrent modulation results in periodic changes in the frequency of theoptical beam produced by the laser diode, which is split by a beamsplitter into two beam paths. The optical measured route is in the firstbeam path, and an optical monostable element is provided in the otherbeam path. Due to the periodic modulation, a frequency difference,corresponding to the heterodyne frequency, always appears between thetwo beam paths. The two partial beams are brought to a beam receiver forinterference after traveling through the measured route. The measuredsignal generated at the beam receiver has a sinusoidal signal shape. Thefrequency of the measured signal is equal to the heterodyne frequency.The information about distance, angle or velocity is contained in thephase angle of the measured signal with respect to a reference signal.The reference signal can also be made available optically in an opticalreference route. In this case, the two partial beams are brought to anadditional beam receiver for interference after traveling through theoptical reference route. The optical reference route may be omitted ifthe reference signal is derived electrically from the modulation signalof the laser diode.

European Patent Application No. 0 729 583 describes a device formeasuring the phase difference between two electric signals occurring ina heterodyne interferometer. Each predefined edge of a first signal andeach predefined edge of a second signal trigger counting sequences thatallow the determination of the phase difference from multiples of 360°and high-resolution measurement of the angle from 0° to 360°. Noinstructions are given regarding the pre-processing of the signals sentto the known device.

The object of the present invention is to provide a circuit forpre-processing signals occurring in a heterodyne interferometer.

SUMMARY OF THE INVENTION

The circuit according to the present invention has the advantage that aperiodically occurring sign change between a reference and a measuredsignal does not affect the determination of the phase difference. Such aperiodically occurring sign change arises due to the modulation of theradiation source with the predefined modulation signal. A phase jumpoccurs if the sign of the slope of the frequency variation changes.

The circuit according to the present invention assures that thereference signal and the measured signal, between which the phasedifference to be measured occurs, have the same heterodyne frequency andthat the heterodyne frequency is defined as an integer multiple of themodulation frequency of the radiation source. This can be achieved bysuitably dimensioning an optical monostable element or by appropriatelydefining the amplitude of the modulation signal.

A gate signal, whose period corresponds to the modulation period, isderived from the modulation signal. Using the gate signal, the referencesignal is filtered by multiplication so that those signal components ofthe reference signal having the same phase sign are forwarded. Themeasured signal is also filtered by multiplication using the gate signalso that only those measured signal components having the same sign areforwarded. The gate signal filtered signals can be sent to a phasecomparator known from the related art, which determines the phasedifference, from which the result can be derived.

Further signal processing is simplified when the gate signal filteredsignals are each sent to one of the band pass filters tuned to theheterodyne frequency. The band passes reconvert the unsteady gate signalfiltered signals into continuous signals, which can be further processedin a simple manner using analog circuit technology.

The gate signal filtered or band pass filtered signals are eachpreferably sent to a mixer, which allows the signal frequency to bereduced in order to allow even simpler further signal processing.

The gate signal or band pass filtered or down-mixed signals arepreferably subjected to low-pass filtering. Low-pass filteringeliminates undesirable signal components, which may appear in the mixer,for example, and further reduces the noise signal components that mayaffect the signals.

Further signal processing is simplified if the signals are sent to acomparator, so that the signals are available as digital signals, whichcan be processed by a digital phase comparator.

A laser diode allowing simple modulation of the optical beam throughmodulation of the injection current is preferably provided as theradiation source in the heterodyne interferometer. A network allowsnon-linearities to be compensated and thermal time constants to be takeninto account in the laser diode. The network allows the laser diodeinjection current to be influenced so that the heterodyne frequency isconstant over a longer period. With these measures, the time windowdetermined by the gate signal can be brought closer to the theoreticallymaximum value, which is equal to 50% of the modulation period.

In an advantageous embodiment of the present invention, the referencesignal is derived from the modulation signal. With this measure, theoptical reference route and an additional beam receiver, which would beotherwise required, are no longer necessary.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a circuit according to the presentinvention.

FIG. 2 shows part of an optical route.

FIG. 3 shows a modulation signal.

FIG. 4 shows a signal variation occurring in the optical route.

FIG. 5 shows a reference signal and a measured signal.

FIG. 6 shows signal variations occurring in the circuit according toFIG. 1.

DETAILED DESCRIPTION

FIG. 1 shows a block diagram of a signal processing circuit for areference signal A occurring in a heterodyne interferometer and ameasured signal B. A clock signal T generated by a quartz oscillator 10is sent to a first frequency divider 11, a second frequency divider 12,and a phase comparator 13. An output signal 14 of first frequencydivider 11 is sent to a modulator 15 and a gate signal circuit 16. Anoutput signal 17 of modulator 15 goes to a laser diode 19 as amodulation signal M(t) after passing through a network 18.

The laser diode 19 emits an optical beam 20 to an optical route 21 whichgenerates a measured beam 22 and a reference beam 23.

The measured beam 22 impinges on a first beam receiver 24 which emitsthe measured signal B. The measured signal B passes through a first gatesignal circuit 25 and goes, as gate signal filtered measurement signalD, to a first band pass filter 26 which emits a band pass filteredmeasured signal E. After passing through a first mixer 27, thedown-mixed measured signal F goes to a first low-pass filter 28 whichsends a low-pass filtered measured signal G to a first comparator 29.

Reference beam 23 is detected by a second beam receiver 30, whichgenerates reference signal A, which, after passing through a second gatesignal filter 31, is sent to a second band pass filter 32 as gate signalfiltered reference signal H. Reference signal 1, band pass filteredafter exiting second band pass filter 32, goes to a second mixer 33.Down-mixed reference signal J, exiting second mixer 33, goes, afterlow-pass filtering in a second low-pass filter 34, as low-pass filteredreference signal K to a second comparator 35.

The two gate signal filters 25, 31 receive a gate signal C generated bygate signal circuit 16. The two mixers 27, 33 receive an output signal36 of second frequency divider 12. The two comparators 29, 35 output afirst input signal L and a second input signal N to phase comparator 13.

FIG. 2 shows a portion of optical route 21 illustrated in FIG. 1.Optical beam 20 output by laser diode 19 goes, after passing through abeam splitter 40, as first optical beam 41 and second optical beam 42 tooptical route 21, not shown in detail. Second optical beam 42 passesthrough an optical monostable element 43.

FIG. 3 shows an example of modulation signal M(t) as a function of timet. An at least approximately triangular signal shape is shown with theperiod marked as T_(m).

FIG. 4 shows the optical frequencies F(t) of the two optical beams 41,42. F1(t) denotes the frequency of first optical beam 41 and F2(t)denotes the frequency of second optical beam 42. A frequency difference,denoted as heterodyne frequency Fh, occurs between the two frequenciesF1(t) and F2(t) at any given time. After a time offset Dt, the twooptical beams 41, 42 attain the same frequency.

FIG. 5 shows reference signal A and measured signal B each as a functionof time Tm.

FIG. 6 shows measured signal B, gate signal C, gate signal filteredmeasured signal D, band pass filtered measured signal E, and first inputsignal L each as a function of time Tm. Gate signal C has a windowperiod To.

The circuit according to the present invention operates as follows:

The part of optical route 21 shown in FIG. 2 is part of a heterodyneinterferometer, in which first optical beam 41 and second optical beam42 travel through a reference route and a measured route and are broughtto beam receivers 24, 30, respectively, for interference. The injectioncurrent of laser diode 19 is modulated using modulation signal M(t). Asuitable modulation signal M(t) is shown in FIG. 3, according to which atriangular signal, for example, is appropriate. Instead of thetriangular signal shown in FIG. 3, a saw tooth shaped signal can also beprovided. The period of modulation signal M(t) is Tm. In addition toinfluencing the radiation power of laser diode 19, the frequency ofoptical beam 20 output by laser diode 19 is modified. From beam 20generated by laser diode 19, beam splitter 40 produces first and secondoptical beams 41, 42. In contrast with first optical beam 41, secondoptical beam 42 has passed through optical monostable element 43.Optical monostable element 43 can be implemented, for example, as anoptical bypass in air, in a glass fiber, or, for example, in a prism.Due to the periodic modulation, both optical beams 41, 42 have, at anytime, a frequency difference, which is referred to as heterodynefrequency Fh.

Using FIG. 4 it is explained below how heterodyne frequency Fh isobtained. FIG. 4 shows optical frequencies F(t) as a function of time t.Optical monostable element 43 delays second optical beam 42 by a timeoffset Dt with respect to first optical beam 41. Assuming that thefrequency of optical beam output by laser diode 19 is constantly changedby modulation signal M(t), at any time the same frequency difference,namely, heterodyne frequency Fh, occurs between the two optical beams41, 42. The linear frequency rise and the suggested linear frequencydrop shown in FIG. 4 is achieved through the triangular signal shape ofmodulation signal M(t) as shown in FIG. 3.

Optical route 21 contains, for example, a fixed reference route and avariable measured route. The measured route is changed by the motion ofa measured object. The reference route is used for generating thereference phase. Each of the two beam receivers 24, 30 must receive bothfirst and second optical beams 41, 42. Only then is it guaranteed thatelectric signals with heterodyne frequency Fh, instead of the opticalsignal frequency, which cannot be easily analyzed using electricalmeans, are to be analyzed.

Reference signal A and measured signal B, which are shown in FIG. 5 as afunction of time, plotted in the measuring units of period Tm ofmodulation signal M(t), have a sinusoidal shape, with the essentialinformation being contained in the phase angle.

If reference signal A is not available, the reference signal can also beobtained electrically from modulation signal M(t). Interference thataffects optical route 21, such as a varying ambient temperature, can betaken into account through electric effects, if needed, in the electricimplementation of reference signal A.

The signal processing circuit according to the present invention allowsthe phase difference between reference signal A shown in FIG. 5 andmeasured signal B to be determined. At any time, the two signals A, Bhave the phase difference to be determined. The prerequisite is thatreference signal A and measured signal B have the same heterodynefrequency Fh and that the heterodyne frequency be established as aninteger multiple of the modulation frequency. This can always beachieved by suitably dimensioning optical monostable element 43 and/orby appropriately establishing the amplitude of modulation signal M(t).

The two signals A, B suffer phase jumps at times Tm/2, which are causedby changes in the slope of modulation signal M(t). The phase jumps makeit difficult to determine the phase difference. The circuit according tothe present invention allows signal processing resulting in first andsecond input signals L, N, which can be processed by a conventionalphase comparator. Since both signals A, B are processed in an identicalmanner, in the following only the processing of measured signal B willbe described using block diagram shown in FIG. 1 and the signal curvesas a function of time shown in FIG. 6.

Quartz oscillator 10 provides clock signal T, which has a frequency of afew MHz, for example, 32 MHz. All the other required signals are derivedfrom clock signal T. First divider 10 divides clock signal T by 32, forexample, so that the frequency of output signal 14 of first divider 11is one MHz. The inverse of the frequency of output signal 14 of firstdivider 11 determines period Tm of modulation signal M(t) generated inmodulator 15. Modulator 15 outputs output signal 15 to network 18, whichgenerates modulation signal M(t). The distortion of the laser diodecurrent, which can be specified by preferably linear network 18 makes itpossible to compensate for non-linearities or, for example, thermal timeconstants of laser diode 19. Thus it can be achieved that heterodynefrequency Fh is constant over an extended period.

Measured beam 22, which is detected by first beam receiver 24, havingheterodyne frequency Fh due to the interference of the two optical beams41, 42, is detected by first beam receiver 24, processed and madeavailable as measured signal B. Measured signal B is multiplied in firstgate signal filter 25 by gate signal C and then output by gate signalfilter 25 as gate signal filtered measured signal D. Gate signal C isshown in the second curve of FIG. 6. Gate signal C is obtained fromoutput signal 14 of first divider 11. Gate signal C is a square signalhaving period Tm and window period To. FIG. 6 is based on a pulse dutyfactor of approximately 50%. Gate signal C is derived, for example, viamonostable flip-flops from output signal 14 of first divider 11. Gatesignal circuit 16 may contain two monostable flip-flops for thispurpose. With the time constants of the two monostable flip-flops, therelative position of the resulting signal window and its relativeduration To can be specified. Gate signal C eliminates those signalcomponents from measured signal B that have the same phase sign atconstant heterodyne frequency Fh. By accurately determining the windowstart and duration To of the window, gate signal C also contributes tosuppressing interference signals, which may arise in laser diode 19 whenthe slope of modulation signal M(t) changes in actual use.

Gate signal filters 25, 31 can be implemented as analog switches withthe switching function being initiated by gate signal C. From theelectric point of view, gate signal filtering corresponds to multiplyingmeasured signal B by gate signal C. The result is gate signal filteredmeasured signal D, which is shown in the third curve of FIG. 6. Gatesignal filtered measured signal D only contains those components ofmeasured signal B, which have the same phase sign.

First band pass filter 26 provided next, whose central frequency istuned to heterodyne frequency Fh, leads to interpolation of gate signalfiltered measured signal B. The gaps resulting from gate signalfiltering are thus eliminated. After a limitation, which can beperformed by a comparator, for example, first input signal L can bederived from band pass filtered measured signal E. Signal processing,however, can be simplified using first mixer 27 shown in FIG. 1. Bandpass filtered measured signal E, whose fundamental frequency isheterodyne frequency Fh, is converted in first mixer 27 into down-mixedmeasured signal F. First mixer 27 receives output signal 36 of seconddivider 12, which has signal components with the frequency required forthe mixer.

After passing through first low-pass filter 28, which essentially ridsdown-mixed measured signal F from undesirable mixed signal componentsand from noise, low-pass filtered measured signal G is converted intofirst input signal L in first comparator 29, which compares low-passfiltered measured signal G with a predefined threshold value. Firstinput signal L is shown in curve 5 of FIG. 6, first input signal L beingdirectly derived from band pass filtered measured signal E. Down-mixingin first mixer 27 would increase the period in comparison with firstinput signal L shown in FIG. 6.

Signal processing of reference signal A is fully identical to theabove-described signal processing of measured signal B. Second inputsignal N is the result of the processing of reference signal A. Phasecomparator 13, which is described in detail in the related art describedabove, for example, determines the phase difference that exists betweenthe two input signals L, N. The phase difference is a measure of thedistance or angle in optical route 21, whose change over timecorresponds to a velocity or an angular velocity.

What is claimed is:
 1. A signal processing circuit for a referencesignal and a measured signal, the reference signal occurring in aheterodyne interferometer, the signal processing circuit comprising:amodulator controlling a particular frequency of an optical beam using amodulation signal which has a predetermined modulation period, theoptical beam being emitted by a radiation source; a first arrangementderiving a gate signal from the modulation signal, the gate signalhaving a period which is equal to the modulation period; a secondarrangement receiving the reference signal and filtering the referencesignal using the gate signal to generate a first filtered signal, thefirst filtered signal including first signal components, each of thefirst signal components having the same phase sign; a third arrangementreceiving the measured signal and filtering the measured signal usingthe gate signal to generate a second filtered signal, the secondfiltered signal including second signal components, each of the secondsignal components having the same phase sign; and a phase comparatorreceiving the first and second filtered signals, and determining a phasedifference between the reference signal and the measured signal, whereinthe reference signal has a particular heterodyne frequency, and themeasured signal has the particular heterodyne frequency, and wherein theparticular heterodyne frequency is an integer multiple of a modulationfrequency of the modulation signal.
 2. The signal processing circuitaccording to claim 1, further comprising:a band pass filter receivingthe first and filtered second signals, the band pass filter being tunedto the particular heterodyne frequency.
 3. The signal processing circuitaccording to claim 2, further comprising:a mixer receiving the first andsecond filtered signals for reducing the particular heterodynefrequency.
 4. The signal processing circuit according to claim 2,further comprising:a low pass filter receiving the first and secondfiltered signals.
 5. The signal processing circuit according to claim 2,further comprising:a plurality of comparators receiving the first andsecond filtered signals, and generating first and second output signals;and a phase comparator receiving the first output signal as a firstinput signal and the second output signal as a second input signal. 6.The signal processing circuit according to claim 1, wherein theradiation source includes a laser diode.
 7. The signal processingcircuit according to claim 6, wherein the laser diode generates a diodecurrent, and further comprising:a non-linear characteristic elementdistorting the diode current.
 8. The signal processing circuit accordingto claim 1, further comprising:a fourth arrangement deriving thereference signal from the modulation signal.
 9. The signal processingcircuit according to claim 1, wherein the gate signal forms a timewindow which has a predetermined duration.
 10. The signal processingcircuit according to claim 1, wherein the gate signal forms a timewindow which has a predetermined start time.